Symbiotic nova

A symbiotic star is an interacting binary star system consisting of a red giant and a hot blue companion, often a white dwarf, embedded in an ionized nebula.

It is characteristic of symbiotic stars, that the spectrum is composed of an emission and an absorption spectrum. As with the cataclysmic variables it comes to the matter transfer to the hot companion, in contrast to these, the two stars are but further apart, so it usually does not come to a matter transfer through the Roche limit, but only to Windakkretion.

The term " symbiotic star" goes back to Paul W. Merrill.

Symbiotic stars, which do not reach the necessary conditions for a permanent thermonuclear reaction and where it comes at irregular intervals to the explosive burning of the accreted matter are called symbiotic novae. Typical representatives are T Coronae Borealis and RS Ophiuchi.

Definition

There are numerous definitions for the class of symbiotic stars. The oldest and still commonly relies on the properties of the composite optical spectrum:

• The spectrum shows the characteristics of a late giant with the spectral classes G, K and M, which are neither a main sequence star or a supergiant. These properties include the absorption lines of CA I, CA II, Na, I, Fe, I, H ​​2 O, CN, CO, TiO, VO, and others.
• In addition, shows the star's spectrum emission lines of hydrogen or helium, and either further emission lines with an ionization energy of more than 20 eV (eg O III) or an AF continuum with absorption lines of ionized metals easily.

The General Catalogue of Variable Stars defined as symbiotic star Z And- star. This is to close binary systems consisting of a hot star, a star with a late spectrum and excited by the hot star extended envelope. The changes in brightness can reach up to 4 mag. The class of symbiotic stars is described very heterogeneous.

Notwithstanding describes Joanna Mikolajewska symbiotic star transferred as interacting binaries consisting of a developed giants ( a red giant or a Mira star), which ground to a hot and luminous white dwarf.

Subdivisions

The heterogeneous group of symbiotic stars is divided according to various criteria.

Infrared spectrum

In the infrared can be distinguished:

• S- systems, which account for about 80 % of the symbiotic stars and show in the infrared spectrum only a stellar photosphere with an effective temperature of 3000 to 4000 K
• D- systems that show next to the reddened spectrum of Mira star signs of about 1000 K warm dust shell
• D' systems that accommodate unlike D systems no Mira- star, but an F to K giants

After the hot blue companion

The hot blue companion of the red giant, which releases the ionizing radiation can take the following forms:

• Main sequence star, as in the case of SS Lep
• A white dwarf, as in the majority of symbiotic stars of the case
• A neutron star, as in GX1 4 = V2116 Oph. The ionizing radiation and the emission lines are caused by an accretion disk around the neutron star. The related term symbiotic X-ray star (English symbiotic X -ray binary ), however, describes a low-mass X-ray binary star whose companion is a red or yellow giant, regardless of the presence of emission lines in the optical spectrum.

By type of accretion

In this classification, a distinction is the manner in which the blue companion is supplied from the red giant matter.

• Windakkretion. In the accretion also known as Bondi - Hoyle accretion of the blue companion gathers matter with the help of his gravity from the undirected emitted by red giant stellar wind. This is the case with most symbiotic stars.
• Flow via the Roche limit. In a binary system, there is a maximum radius of the red giant can take. Expands, the star above this limit, matter flows through the inner Lagrangian point to the companion. The case possible mass flow is considerably greater than in Windakkretion.

By type of hydrogen burning

The accreted hydrogen and possibly helium can almost permanently burned on the surface of the white dwarf or in the main sequence star. Will not the necessary temperature, pressure and flow of matter reaches for a permanent combustion, so it comes to an explosive combustion, a thermonuclear runaway. Such binaries are also called symbiotic novae.

Variability

All symbiotic stars are among the variable stars. The brightness changes can be assigned to different causes:

• Bedeckungsveränderlichkeit when the bright blue component of the Earth is behind the red giant. This type of variability is suitable for the analysis of the geometrical dimensions of the binary system.
• Reflection effect. The radiation of bright blue companion heated to him facing side of the Red Riesens and leads to a color and brightness change with the period of the orbital period.
• Variability due to the ellipsoidal shape of the red giant's, which arises due to the proximity to the blue companion. Also this variability varies periodically with the orbital period of the binary system and can be separated only in the infrared from the other forms of variability.
• Pulsations of the Red Riesens that occur either regularly or semi- regularly approximately in the case of Mira stars. The brightness changes occur over periods of months to years
• The rotation period of the Red Riesens can modulate the light curve over starspots or variable intensity of the outflow of matter along the magnetic field disturbances
• Flickering with amplitude of up to 0.5 may be a matter of minutes. The Flickering appears only in symbiotic stars appear with flow over the Roche limit
• Quasi- periodic oscillations, which are probably similar to Zwergnovaoszillationen
• A periodic signal due to the rotation of the white dwarf and the incidence of matter along the magnetic field lines of the white dwarf. The period is of the order of 10 minutes,
• Normal outbursts of type Z And. These outbursts last for months to years and show an increase in brightness up to 4 mag in the ultraviolet. The bolometric brightness is almost constant, but in which there is a decrease in the effective temperature of the blue companion from 100,000 to 10,000 K.
• Symbiotic nova outbursts with Hellikeitsänderungen of up to 10 mag within days to decades

Orbital parameters

The orbital period in symbiotic stars of type S is between 200 and 1000 days, and the Type D with up to 44 years. The tracks are compared to other double stars nearly circular, they have a low eccentricity of almost 0 from. Only the symbiotic stars, with their companions, it is a main sequence star, show a mean deviation from the circular shape. The small eccentricity in symbiotic stars with a white dwarf is a result of a previous common envelope phase (English common envelope ). The white dwarf is previously been a red giant, which has transferred a portion of its atmosphere on the current red giant. Here, the former red giant had extended so that the orbit of the companion at times was within its extended atmosphere. Frictional forces are then fed to a disappearing of the eccentricity and shrinkage of the orbital path.

In general, are the masses of the red giants from 0.6 to 3.2 solar masses. The masses of the blue component are usually between 0.4 and 0.8 solar masses in the classical symbiotic stars, and between 1.1 to 1.3 solar masses in the repetitive symbiotic novae. The mass of a blue main sequence star in a symbiotic binary star system can accept up to 8 solar masses.

The red giant

The spectral type of the red giants in symbiotic binary systems is usually between M3 and M7. This is a very late spectral type compared to the general galactic field for red giants. Continue to show the red giant in the middle a strong stellar wind. It was determined by radio observations at more than 10-7 solar masses per year. A strong stellar wind is a prerequisite for an adequate accretion onto the blue companion and therefore a selection effect. With the stellar wind often stellar masers have been observed as in the OH / IR stars around symbiotic stars. Here are lines of OH, SiO, H2O and CO. In the case of the symbiotic nova V407 Cygni, it was possible to investigate the origin of the maser radiation in detail, since the kinetic energy of the ejected shell has the Nova outburst the maser, which requires a steady stellar wind, interrupted. But three months later the stellar wind of the Mirasterns was restored to the extent that a stellar maser was detected again.

At the very heterogeneous structure of the symbiotic stars, it is not surprising that not necessarily a red giant must be present in a double star system. In the mass- losing companion can be also yellow giants with the spectral types GK or carbon star IPHAS J205836.43 503307.2 like.

The blue companion

The blue companions in a symbiotic binary system shows the ultraviolet frequently have a temperature of more than 100,000 K at 100 to 1000 times solar luminosities. In the Hertzsprung -Russell diagram, the position of the central stars of planetary nebulae with those of symbiotic stars overlaps. The high luminosity can be not only a result of accretion onto the white dwarf, since this would require an accretion rate of at least 10-6 solar masses per year. This would be higher than the total non-directed stellar wind from the red giant. Therefore, the high luminosity is probably the result of a permanent hydrogen burning on the surface of the white dwarf. The luminosity of the accretion disk is likely to play only a minor role, with the exception of symbiotic stars with a neutron star. Another exception are probably symbiotic stars with a high-mass white dwarf as a blue companion. With a mass close to the Chandrasekhar limit hard X-rays and Flickering can be detected with a large amplitude in the resting light. Both phenomena are attributed to variations in the accretion rate and are directly a result of the liberated in the accretion potential energy. The blue companion is also the source of the classic Z -And- outbursts and nova outbursts.

The energy stored in an accretion disk mass is likely to be between 10-5 and 10-3 solar masses. Of this sum, between 50 and 80 % of the white dwarf, while the rest flows through a vertical wind from the accretion disk. Overall, the white dwarfs are expected in a few million years of symbiotic phase accrete only 0.1 solar masses, which matter is ejected to a considerable proportion on nova outbursts back into the interstellar space.

Classical symbiotic outbursts

The outbreaks of type Z And take months to years and show an increase in brightness up to 4 mag in the ultraviolet. The bolometric brightness remains approximately constant. However, there is a decrease in the effective temperature of the blue companion from 100,000 to 10,000 K and consequently to a shift of the electromagnetic radiation from the deep ultraviolet to the optical spectral range. Furthermore, increases the strength of the high-excitation emission lines, probably expands the accretion disk and forms a bipolar outflow of the white dwarf or the accretion disk. Parallel to the increase in optical brightness increases, the hard X-rays to which probably is caused by bremsstrahlung, if the matter collides from the bipolar outflow with the stellar wind of the red giant's. The ionization zone around the symbiotic binary star expands during an outbreak.

The outbreak is explained as a consequence of increased accretion rate due to thermal instability of the accretion disk, which leads to an expansion of the zone of hydrogen burning and thus to the formation of a A to F- pseudo- photosphere. The biggest problem for this model is the short distance between the outbreaks, which may be only a few years. During this period, the emptied accretion disk may have not filled in Windakkretion again.

Symbiotic stars with a neutron star show no outbreaks in the optical spectrum. Your outbreaks take place almost exclusively in the field of hard X-rays and are also the result of instability of the accretion disk, similar to the model of dwarf novae outburst. The X-ray radiation is produced upon impact of the accreted matter on the crust of the neutron star, and this interpretation is supported by accelerated rotation of the Röntgenpulsars after the end of the outbreak. Of symbiotic stars with a main sequence star as the blue component no large eruptions are known.

Symbiotic novae

A nova is the result of thermonuclear runaways ( an explosive ignition of thermonuclear reactions ) on the surface of a white dwarf. The result of the sudden onset of hydrogen burning is a steep increase in brightness, the formation of a strong stellar wind connected to the output of a shell, an infrared excess due to dust formation at some distance from the Nova through the sloughed material and evidence of soft X-ray source by waste of optical brightness. The supersoft X -ray source is visible when the produced during hydrogen burning X-ray radiation is not absorbed because the expanding envelope has become transparent.

Symbiotic novae are different from the classical novae at first only by the mass- donating companion of the white dwarf, which is a red giant in classical novae a main sequence star or subgiant and symbiotic novae. As a result, the amplitude of the outbreak of the symbiotic novae is apparently smaller, because the red giant contributes more light to rest brightness. Symbiotic novae fall into the repetitive symbiotic novae and the extremely slow novae. The repetitive symbiotic novae are fast novae with a brightness increase within days and they return within a few months back to rest brightness. The masses of the white dwarfs lie between 1.1 and 1.3 solar masses, and therefore the conditions for re- ignition of a thermonuclear runaways are given again after a few decades. Your accretion rate is about 10-7 solar masses per year.

The very slow symbiotic novae show an increase in brightness over months and take years to decades (AG Peg circa 100 years) to return to the calm brightness. The white dwarfs have a mass of less than 0.6 times the sun. In these novae much of the accreted hydrogen is lost by the stellar wind due to the slow reaction rate at the surface of the white dwarf. The eruptions of symbiotic novae such as has been demonstrated in RS Oph and V407 Cyg -energy gamma radiation, in contrast to classical novae. Again, this is interpreted as a consequence of the formation of a shock front between matter from the Nova outburst and the stellar wind of the red giant's.

Repeating symbiotic novae are candidates for the precursors of supernovae of type Ia. These supernovae are standard candles light of cosmology and led to the discovery of the accelerated expansion of the universe. Although it is generally accepted that supernovae of type Ia caused by the collapse of a CO white dwarf after exceeding the Chandrasekhar mass limit, it has so far not managed a precursor of a supernova of this type prove nor show a development process that is not in contradiction with other observations available. Since repetitive symbiotic novae white dwarfs with masses near the Chandrasekhar mass limit accommodate, they are ausssichtsreiche candidates. However, it is not clear whether the white dwarf does not lose more mass during the outbreaks is obtained by accretion. There is an unusual symbiotic star with the designation J0757, which shows between outbreaks no sign of symbiotic activity but only the spectrum of a red giant's. A flare in the 1940s, with a ten-year period with no evidence of mass outflow is interpreted as a quiet hydrogen burning on the surface of the white dwarf. This type of symbiotic stars could evolve into a supernova of type Ia, since in these the mass of the white dwarf increases. But you are too rare to make a significant contribution to the observed rate of 0.003 Ia supernovae per year to deliver in the Milky Way. In contrast, evidence of several circumstellar envelopes of gas and dust have been found in the light curve and the spectra of the supernova PTF 11kx Type Ia. The speed with which these cases move, it's too fast for a stellar wind and too slow to originate from the supernova itself. The distance between the shells in combination with the expansion velocity can nova outbursts with a distance of several decades between the eruptions as the most likely source of the gas and dust shells appear. Such a short distance between Nova eruptions and the presence of a continuous, the stellar wind of a red giant resembling component in the circumstellar envelope around the supernova indicate a symbiotic nova. However, supernovae of type Ia with the 11kx observed as with PTF properties are very rare and are therefore likely to be responsible for a maximum of 10% of all cases of this supernova group.

Symbiotic fog

The ionized nebula around a symbiotic star is called a symbiotic nebula (English symbiotic nebula ). It differs in spite of a different evolutionary history in many properties indistinguishable from those planetary nebula, because the blue component of symbiotic binary stars in the Hertzsprung -Russell diagram planetary nebula is located at the position of the central star. It may therefore be assumed that many planetary nebulae are misclassified.

Symbiotic nebulae are almost all asymmetric and show at least 40% a bipolarity. As a source of bipolarity a binary nature of the central star is assumed in both symbiotic and planetary nebulae. The electron density is 106-1010 per cubic centimeter significantly higher and more in line with the solar corona. The electron temperature with 10,000 to 80,000 K is comparable to the planetary nebula. From spectral analyzes chemical abundances were determined in symbiotic nebulae and the origin of the gas is recirculated in the fog on the red giant. From the blue companion for hydrogen burning prozessiertes plasma is emitted by the stellar wind and partly Jets in the fog. However, this source is present in the resting phases a subordinate role both in the amount of inserted material as well as ionization source. It was only during the classical symbiotic outbursts is the kinetic energy of the stellar wind of the blue companion an important energy source in the symbiotic nebula.

The stellar wind, resulting in the formation of the symbiotic nebula, is also a source of soft X-rays from the symbiotic systems. In the region collided in the stellar wind of the red giant's with the outgoing from the blue companion wind, the gas is heated to temperatures that lead to thermal emission of up to 2.4 keV. The brightness is from 1030 to 31 erg / s requires a wind speed of the blue component of some 100 km / s, as is also derived from the optical spectra.